The last thing you might expect to see Jim Broadbent doing on the big screen is snorting cocaine.

One of Britain’s finest actors, the 60-year-old looks like a well-dressed unassuming gentleman – and as far from a drug addict as possible.

Yet in his latest film, Perrier’s Bounty, Broadbent has to snort coke up his nose and gulp instant coffee straight from the jar in his role as eccentric Jim McCrea, who is convinced he will die if he falls asleep.

“Jim has had a message and had a visitor, in the form of the Grim Reaper, and he’s absolutely convinced he’s just got to keep awake forever,” he explains.

“Part of that involves getting as much cocaine up his nose as he possibly can. That’s something I haven’t done on screen before.”

The ’cocaine’ was actually glucose powder – which, according to the Lincoln-born actor “has its own mild energising effect” – while the ’instant coffee’ was, in fact, chocolate biscuits.

“They used chocolate biscuits ground up so it looked very convincing. Crunched up chocolate biscuits aren’t pleasant in itself, and when you have the sixth take...” he adds with a grimace.

Jim is the estranged dad of Michael McCrea (played by Cillian Murphy) and the pair end up going on the run together because Michael owes money to renowned Irish gangster Darren Perrier (Brendan Gleeson).

“It couldn’t be more inappropriate and it couldn’t be more awkward for Michael’s character to have his so-called dad hanging around his neck,” says Broadbent.

The mild-mannered star – who has stamped his reputation by playing goody-goody fatherly roles like writer John Bayley in Iris, for which he received an Oscar for best supporting actor, and Bafta-winning Moulin Rouge scene-stealer Harold Zidler – relished the chance to pistol-whip some baddies.

“I get to hold the gun as well, and hit somebody with it, and point it at a policeman and say, ’Freeze’. I don’t get to do much of that – it’s good fun,” he says, chuckling.

Broadbent reveals that fun was part of the reason he said yes to the film.

“It was a great script to get. It’s very funny from the word go,” he says.

“There’s something alive about it. It’s a cracking role. It was irresistible, the script was great and the character was so delicious. The idea of his predicament is very attractive for an actor. Somebody who knows he’s going to die, so he has to stay awake for as long as possible. All that tension sets up the jokes.”

One of the challenges of the role was perfecting his Irish accent.

“I loved the character so much even if it meant having an Irish accent and sitting amongst lots of genuine wonderful Dublin accents from all the Irish actors.

“It’s quite a tricky accent and I was a little bit apprehensive but I had fun doing it, and I think it works to some extent,” he says.

Broadbent has a lot of scenes with his on-screen son and his neighbour Brenda (Jodie Whittaker), who joins them on the run after killing one of Perrier’s goons.

“It was good fun though because the three of us – Cillian, Jodie and I – were thrown together a lot,” he recalls.

“We didn’t have a lot of time to get together, we just kicked off. The three of us got on really well from the word go. We had such a laugh when we were sitting in the car for hours and hours on end, so we were able to have that fun, banter and jokes, then we could all switch to being scared and terrified easily.”

Despite acting in films since 1978, Broadbent admits he still learnt something on Perrier’s Bounty.

“Each film has a different requirement, so you’re always learning something different on every film,” he says.

“With this one, you’ve got to pitch it at a different level to get the comedy and drama, so you’re all working together to find that. I’ve never done exactly this mixture of comedy and thriller so you’re learning all the time.”

He has also finished shooting Another Year, to be released later this year, which sees him reuniting with acclaimed filmmaker Mike Leigh.

“I’ve worked with him so often. In 1979, I did a play with him so it’s been 30 years. He’s been very consistent with his way of working and stuck to his guns,” he reveals.

His next big-screen outing will see him reprise his role as Potions teacher Horace Slughorn in the final two instalments of Harry Potter, Harry Potter And The Deathly Hallows.

“Harry Potter was terrific – it’s one of these things that will be seen forever,” he reveals.

“It was in its sixth film when I joined, and they had so much money to spend on it – we had five-day weeks, short days because you work around kids’ hours and 30 minutes from London. It’s a really civilised way of working. It’s really pleasant, they do a fantastic job. The kids are all great, there’s no sort of brat pack behaviour.”

Diva-ish behaviour is something Broadbent, who comes from the old school of thought that you should be professional at all times, will not tolerate.

“I don’t like moaning movie stars – there’s no excuse for it, you’re in such a good position. Just shut up!” he asserts.

“Very rarely have I come across starry behaviour. I’ve heard about it but I’ve never actually witnessed it in 50-odd films. I am surprised if it really is as common as reported.”

He continues: “It’s always easier to report and invent bad news but actually, making a film is such a collaborative thing that on the whole, you’ll be embarrassed to behave badly. You’d be holding everyone up!

“I think it must be a fairly unique occurrence. If you choose a good script, then the people involved are going to be doing it for all the right reasons.”

Having played a variety of characters from Bridget Jones’ kindly dad (Bridget Jones’ Diary, Bridget Jones: The Edge Of Reason) to King William IV (The Young Victoria) and Lord Longford (Longford), is there one role that he hankers for?

“No, I never have any dreams about what I want to do. Be careful about what you wish for,” he says, smiling.

“I think other people’s ideas about what I can do are a lot better than what mine are. I loved doing this, and it wouldn’t have occurred to me in a million years to put myself forward for it. I like it when people come up with ideas.”

Broadbent may be approaching retirement age, but plans to carry on acting for a while longer.

“Once you get an award, you start thinking, ’I don’t need to think about careers or if it’s the right movie’, you just do things that you want to do,” he says.

“It’s a nice position to be at, as long as they pay you a bit and get a fair bit of time off. I love what I do.”

Perrier's Bounty will be released in cinemas on Friday March 26.

EXTRA TIME – JIM BROADBENT

James ’Jim’ Broadbent was born in Lincoln on May 24, 1949.

He made his film debut in 1978 with a cameo in Jerzy Skolimowski’s The Shout, and made his television debut the following year.

He has been married to painter and former theatre designer Anastasia Lewis since 1987, and has two stepsons.

He has no desire to be a celebrity: “I merge into the background, me”.

He has won many awards, including a best supporting actor Oscar for Iris, Golden Globe for best actor in Longford and a best supporting actor Bafta for Moulin Rouge.